B767 CREW MEMBER ACTIVATED THE RUDDER TRIM TO FULL NOSE L WITH A FOOT ON THE PEDESTAL.

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INADVERTENT RUDDER TRIM. WHILE CRUISING AT FL390 AND PASSING A WAYPOINT; WE NOTICED THE ACFT TURING TOWARD THE NEXT WAYPOINT. THE ACFT CONTINUED THE TURN PAST THE INTENDED COURSE AT WHICH TIME I THOUGHT THAT THE WAYPOINTS MAY HAVE SOMEHOW GOTTEN OUT OF SEQUENCE. I REACHED UP AND SELECTED HDG HOLD; BUT THERE WAS NO RESPONSE. AS THE BANK ANGLE CONTINUED PAST 15 DEGS; I KNEW THAT SOMETHING WAS NOT RIGHT. I DISCONNECTED THE AUTOPLT; ROLLED THE WINGS LEVEL AND REALIZED THAT THE ACFT WAS OUT OF TRIM IN THE YAW AXES. WE LOOKED DOWN AND NOTICED THAT THE RUDDER TRIM WAS SHOWING 15 UNITS OF L TRIM; AT WHICH TIME WE TRIMMED THE ACFT; RE-ENGAGED THE AUTOPLT; AND CONTINUED TO OUR DEST. AFTER THE EVENT; WE DISCUSSED HOW 15 UNITS OF TRIM COULD HAVE BEEN PUT IN INADVERTENTLY. WE REALIZED THAT JUST PRIOR; THE FO WAS STANDING BTWN THE SEATS WITH HIS FOOT ON THE BACK OF THE CTR PEDESTAL; HE MUST HAVE INADVERTENTLY TOUCHED THE TRIM KNOB. I REALIZE THAT ALL BOEING CENTER PEDESTALS AS WELL AS RUDDER TRIM KNOBS WERE MODIFIED TO PREVENT THIS EXACT SCENARIO AFTER A B737 ACCIDENT IN LGA IN 1989. DURING THIS EVENT; WE DID NOT DEVIATE OFF ALT AND OUR XTRACK ERROR WAS LESS THAN 1 MI.